Animation Nodes Loop Issue
There are three reasons why your node tree don't work as you expect it to. The following three sections explain those reasons in details.

Data Copying

First, please read the Data Copying section in the documentation to understand what we are talking about. You expect all the vertices to move one unit upward, your reasoning is that the condition is always True and this is true. However, notice that your If True input is connected to a Set List Element node that, in turn, is connected to a parameter of the loop. Since that parameter is connected to the If False input of the Switch node, enforced data copying is performed, subsequently, the Set List Element doesn't edit the list, but rather a copy of the list that is only available is the scope of each iteration. By the end of the loop, you never moved any vertices, you moved only one vertex in each iteration in its scope. Cut the connection going to the If False input and you will see the loop works as expected, that's because data is no longer being copied, that is why when deleting the condition branch, everything works as expected.

Instant Overwrite

You are outputting the vertices to the mesh at each iteration and each iteration overwrite the vertices location that was written in the previous iteration, so, the only effective data written is that of the last iteration simply because it will not be overwritten. In that last iteration, the If True input is the original vertices except for the last vertex moved upward one unit which you observed as that at the location (1,1,0), the reason for this is what which we described in the foregoing section. In conclusion, never write data inside a loop! Output the data from the loop then write it.

Infinite Offset

You are getting the vertices locations from the Cube and outputting it to the Cube, upon continuous execution of the node tree, the vertices will start to move to infinity, which, I assume, is not something you want. In conclusion, your source shouldn't be your output, either cache the vertices location or output to a new mesh.